Electro-acoustic transducers



June 5,1956

Filed Oct. 8, 1951 D. E. SKELTON ELECTRO-ACOUSTIC TRANSDUCERS 2 Sheets-Sheet 1 Inventor D.E. S KEL TON A ttorney June 5, 1956 SKELTON 2,748,882

ELE TRO-ACOUSTIC TRANSDUCERS Filed Oct. 8, 1951 2 Sheets-Sheet 2 \l Q Q u Q) (I) k a In j Inventor DE. SKEL TON Altorne y 2,748,882 ELECTRO-ACOUSTIC TRANSDUCERS Dennis Edward Skelton, London, England, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application October 8, 1951, Serial No. 250,217

Claims priority, application Great Britain October 11, 1950 2 Claims. (Cl. 181-31) This invention relates to electro-acoustic transducers.

According to the invention there is provided an electro-- acoustic transducer having a diaphragm and a membrane of a material which is slow to recover its shape after being deformed and which membrane is so mounted relative to the diaphragm that when one vibrates the other vibrates.

The invention will be readily understood from the following description of certain embodiments in the form of carbon microphones, illustrated in the accompanying drawings in which:

Fig. 1 shows a microphone capsule;

Fig. 2 shows a modified microphone capsule together with the associated front cover.

The capsule in Fig. 1 has a main case 1 in the form of a shallow cup with a central boss. The rim of the cup forms the seating for the diaphragm and the boss is hollowed out to form the granule chamber. The hollow bore through the boss has an inwardly extending ringshaped portion 1a mid-way along it axially. The end faces of this ring-shaped portion form annular seatings for front and back electrodes 2 and 3, the inner surface being of roughly conical form with the wider diameter facing the same way as the rim of the cup. The profile is not strictly conical but is curved into an S bend.

The front electrode 2 has a central domed portion and a flat annular outer portion which rests on the front end face of 1a being secured by a ring 7 of lead or similar cold flowing material pressed into the angle of the depression in which the outer rim of 2 is seated. The domed portion faces rearwards (away from the diaphragm seating).

Back electrode 3 is similar to 2 but the dome shaped portion is shallower. It rests on an insulating Washer seated on the other end face of 1a.

The annular portion of 3 is not as wide as the depression in which it fits and is centred therein by means of a back contact member 4 and spacing washer 6 in the following manner:

The back contact member has a cup-shaped central portion and a flat annular outer portion. The rim of the cup shaped portion fits around the corresponding part of the back electrode of which the dome projects into the cup of 4. The annular portion of 4 is also not so wide as the depression in the boss of the case. The insulating spacing washer 6 fits closely inside this depression however and also fits closely around the outside of the cupshaped portion of 4, holding 3 and 4 centred in the depression in the boss of case 1 and out of electrical contact therewith.

Parts 3 and 4 may be pre-assembled as by lugs on the flange of 3 engaging slots in the flange of 4, for instance.

Washer 6 is held in place by a cold flowing seal 7a, similar to 7.

The diaphragm 8 has an outer flat annular portion, a conical portion extending inwardly therefrom and a flat circular central portion from the outer parts of which are nited States Patent ice punched fingers or teeth 8a which are bent back axially to make contact with the inner surface of the dome-shaped portion of front electrode 2. The holes left by the bending back of 8a are covered by a disc 13 which is preferably of some light plastic material stuck on to the flat central portion of the diaphragm.

The vibrations of the diaphragm are conveyed to the front electrode through teeth do by means of friction, as described in U. S. Patent 2,539,488, issued January 30, 1956. The front of the diaphragm is covered by a thin membrane 10 which acts as a moisture screen. Membrane 10 rests on the flat annular portion of the diaphragm and both are clamped on to the seating on frame 1 beneath the outer flange of the perforated front cover plate 9 which is held in place by a short Wide clamping tube 11 one end of which is spun over the cover plate 9 and the other end of which is spun over the back of the case the cup shaped portion of which is stepped to provide a ledge for the purpose. Cover plate 9 is dished away from the membrane 10 adjacent to its outer seating flange but the centre part of 9 is dished inwardly to make light contact with the centre of membrane 10. This enables 10 to be very thin without risk of its fluttering and beating against the front cover. Membrane it) is also in this way lightly pressed against the outer parts of the diaphragm stabilising the contact pressure between 3a and 2.

By the use of a material for the membrane 10 which is slow to recover its shape after being deformed, such as polyethylene, polyvinyl-chloride, certain organic, synthetic rubbers (neoprene for example), silicone inorganic rubbers, and, to a lesser extent, natural rubber, it has been found possible to introduce a useful degree of damping into the acoustic system the principal effect of which is a damping of the main resonance of the vibrating systerm with little loss of efficiency at other frequencies. The reason for this is believed to be that the membrane is tightly coupled to the diaphragm, acoustically and to some extent mechanically also, so as to vibrate with it and, in vibrating, the membrane absorbs energy. At frequencies away from the said main resonance the mechanical impedance of the vibrating system excluding the membrane, is high and the impedance possessed by the membrane by virtue of the said properties of the material of which it is made, is swamped. At the frequency of the said main resonance however, the impedance of the vibrating system excluding the membrane, is low in relation to that of the membrane and the resonance is damped.

The membrane may be made of the above-mentioned materials in sheet form or of fabric such as silk, for instance, impregnated with such materials.

Oiled silk which is commonly used for the moisture screen in electro acoustic transducers, is not suitable.

A polyethylene membrane of the order of two thousandths of an inch thick has been found to give good results in the embodiment of Fig. 1.

Figure 2 shows a similar capsule with corresponding items marked with the same reference numerals as in Fig. l. The only difference is the removal of membrane 10 from beneath the front cover plate 9 and the addition of a similar membrane 16, on the outside of the cover plate clamped in position between the capsule and the mouthpiece cap 17 which carries a washer 13 in a trench 19 cut round the inner edge of the internal hollow in cap 17.

The membrane 16 is not centrally supported as in Fig. 1 and it is advisable to use a thicker membrane (say from 4 to 6 thousandths of an inch thick when polyethylene is used), to avoid flutter and also to provide adequate internal damping, since there is a larger span of material capable of free vibration when the centre of the membrane is unrestrained.

In this embodiment the coupling between the diaphragm 3 and the membrane is acoustic only but the similarity in the behaviour of the two embodiments suggests that the mechanical coupling in the Fig. 1 embodiment is unimportant.

The central perforated part of the cap 17, is surrounded on the side remote from the diaphragm by a wall defining a small cup. The wall is cut away on the outside surfaces to form a thin lip around the outer edge of the shallow cup and the outer surface of the wall is of concave sectional form.

The shallow cup acts as an elementary horn, collecting sound from the speakers voice and the concave outer surface deflects nose breath from the sound inlet holes.

In both embodiments, the case 1 is pierced with a hole behind the diaphragm, which is filled with a loosely fitting rivet 15 providing a minute air leak which is 511111- cient to equalise the pressure on the front and back of the diaphragm so as to compensate for Changes of barometric pressure but the leak is too small to affect the acoustic system to any noticeable extent. Both embodiments have a contact ring 14 spun over the back face of the boss of the case 1 to facilitate electrical contact between the case 1 and contact members carried by the mounting into which the capsule is intended to fit. This contact ring 14 provides for electrical connection to the front electrode. The centre contact 4- engages a contact member in the said mounting providing electrical contact with the back electrode.

In an alternative arrangement (not shown), the ring 14 is dispensed with and two terminals are used for direct connection of the wires, one mounted on the main case 1 and the other on the domed portion of the back contact member 4.

While the principles of the invention have been described above in connection with specific embodiments and particular modifications thereof, it is to be clearly understood that this description is made by way of example and not as a limitation on the scope of the invention.

What we claim is:

1. An electroacoustic transducer having a diaphragm, a moisture proof damping membrane of a material which is inherently slow to recover its shape relative to said diaphragm after being deformed in response to the vibrations of said diaphragm, which membrane is mounted in front of said diaphragm and tightly coupled thereto acoustically and mechanically, and an apertured plate covering the diaphragm and the membrane, said plate having a dished central portion making contact with the center of the membrane.

2. An electro-acoustic transducer as claimed in claim 1 in combination in which the membrane has a thickness of the order of two thousandths of an inch.

References Cited in the file of this patent UNITED STATES PATENTS 1,759,387 Bateholts May 20, 1930 1,815,987 Peterson July 28, 1931 2,162,270 Mott June 13, 1939 2,345,078 Ueberschuss Mar. 28, 1944 2,351,400 Carlisle June 13, 1944 2,520,646 Mott Aug. 29, 1950 2,540,498 Tallman Feb. 6, 1951 2,609,458 Vigren et al. Sept 2, 1952 2,646,853 Pocock July 28, 1953 FOREIGN PATENTS 425,186 Great Britain Mar. 8, 1935 

